1. Field of the Invention
The present invention relates to a load pump type of voltage generator circuit and, more particularly, to a load pump designed to be used in a non-volatile memory working with a low supply voltage.
2. Description of Related Art
Load pumps are commonly used in integrated circuits for the production of a voltage greater than an initial voltage, wherein the initial voltage generally corresponds to the supply voltage of the integrated circuit.
The voltage produced by a load pump is obtained by the transfer of electric charges by pumping these charges into a plurality of series-connected capacitors that are insulated from one another by transistors or diodes. The pumping is driven by appropriate driving signals. To carry out this transfer, the load pump has elementary pumping cells comprising at least one capacitor and one transistor or one diode. The driving signals are usually two in number if the capacitors are insulated by diodes and four in number if they are insulated by transistors. The structure of such load pumps is well known and is described in the U.S. Pat. No. 5,077,691.
For an integrated circuit, it may sometimes be necessary to produce several levels of voltage. For example, in the case of a non-volatile memory with a 1.8 volt supply, it is necessary to produce a voltage of about 5 volts to read the cells of the memory and a voltage of about 10 to 12 volts to write in these cells.
The wide range of voltages to be provided for the circuit generally leads to a great increase in the number of load pumps. Accordingly, there is a need for a load pump type of voltage generator device that overcomes this shortcoming.